There are no Ireland wide census records surviving before 1901. Those from 1861 to 1891 were destroyed by the British Government -- the first two for confidentiality reasons it's believed, the latter two were used by the British Government for pulp during WWI.

Earlier census records before 1861 were largely destroyed in a fire at the Fourt Courts Building in Dublin during the Irish Civil War in 1922. Civil registration eg. vital records started in Ireland in 1864 for all with just Protestant marriages beginning in 1845.

You need to research the emigrant's civil and church records in the US and their children's to find specifically where they were born in Ireland, their parents names and their dates of birth. The US 1900 census includes month and year of birth. There's a list of the names of the towns and townlands in the 32 counties in Ireland at the Ireland Townland Database http://www.seanruad.com and pick any county of interest from the drop down menu.

The records to search in the US to find the town/townland, and county of birth, parents names and date of birth include the emigrant's civil and church marriage records, civil and church death records, obituary, cemetery record, tombstone, will and US Naturalisation reords esp. the first papers or declaration of intent which was more detailed than the final citizenship records.

Eilis O'Hara